It is well known and realized that sulfur oxides provide atmospheric pollution and that coals, as well as oils and gaseous fuels, contain sulfur compounds such that as the fuels are burned there is sulfur dioxide (SO.sub.2) and some sulfur trioxide (SO.sub.3) as resulting gaseous products of combustion in the stack gases. It is also known that different types of stack gas treating operations have been tried and are being used with varying degrees of success. In any event, it appears that one of the least expensive, and quite efficient processes, for treating effluent gases involves the use of lime and/or limestone powder in an aqueous slurry to scrub the gas stream and effect a removal of a major portion of the SO.sub.2 -- SO.sub.3 components. The scrubber system is also of advantage in being able to remove any fly ash that may be remaining in the effluent stream reaching the scrubber zone after passage through an electrical precipitator or through a centrifugal type separator system. There are, in addition, certain sulfur compound removal systems which can involve a conversion of the compounds to sulfur and the recovery of substantially pure sulfur; however, such process systems can be quite complex and expensive.
In any case, although lime and ground limestone can be relatively cheap treating materials in comparison to most chemicals and reagents, it is realized that it is of advantage to minimize the amounts to be used and to be "thrown away", as well as of advantage to minimize the sludge concentrating equipment and the handling problems with respect to withdrawn materials. Thus, it may be considered a particular object of the present invention to obtain a more readily precipitatable material and/or a more usable material by effecting a conversion of calcium sulfite in the scrubber bottoms to calcium sulfate.
It is also an object of the present invention to provide a system which utilizes ozone to speed up and enhance the conversion of calcium sulfite to calcium sulfate.
In a corollary aspect, it is an object of the invention to improve the settling characteristics of the withdrawn slurry-sludge material by conversion of the CaSO.sub.3 to CaSO.sub.4 and permit smaller sludge handling equipment than would otherwise be required. Still further, it is actually contemplated that recovered CaSO.sub.4 may be converted to gypsum (CaSO.sub.4.2H.sub.2 O) to be sold as a useful material of construction and/or that it can more easily be stabilized to a relatively hard material for use as landfill.
In one embodiment, the present invention provides a method for effecting the calcium slurry scrubbing and desulfurizing of a sulfur-containing flue gas stream in a manner to minimize the bulk of resulting calcium-sulfur compounds, which comprises the steps of: (a) substantially removing entrained particulates from the flue gas stream, (b) passing the resulting stream to a scrubbing zone and effecting the contacting thereof with an aqueous slurry containing a calcium source, (c) discharging a resulting scrubbed flue gas stream with reduced sulfur content from said scrubbing zone, (d) also withdrawing from said scrubbing zone at least a part of the bottoms slurry containing resulting calcium sulfite and passing such calcium sulfite-containing slurry to a separate ozonation zone, (e) passing an ozone containing gaseous stream into contact with the slurry material in the ozonation zone to effect a conversion of a major portion of the calcium sulfite in the bottoms slurry to calcium sulfate, and (f) withdrawing a resulting calcium sulfate-containing sludge from said ozonation zone.
In another embodiment, the present invention provides in combination with a system having a scrubber means for scrubbing stack gases with a calcium-containing aqueous slurry to remove entrained sulfur compounds, the improved apparatus system which comprises: (a) means for withdrawing a controlled flow of calcium sulfite-containing bottoms slurry from the scrubber means and discharging such slurry into an aerator-ozonator chamber, (b) converter means for producing an ozone-containing stream, (c) conduit means for passing said ozone-containing stream to said aerator-ozonator chamber, (d) gas distributor means connective with said chamber to distribute the ozone-containing stream upwardly through the latter and into contact with the slurry stream also being charged to said chamber, and (e) additional conduit means to withdraw a resulting slurry stream containing calcium sulfate from said aerator-ozonator chamber.
In connection with the scrubbing of the stack gas stream with lime and/or powdered limestone slurry, it is to be realized that powdered limestone may contain some magnesium carbonate and also that there may be utilized powdered dolomite which will contain better than 5% of magnesium carbonate in combination with the calcium carbonate. Thus, in addition to resulting calcium sulfite in the drawn slurry bottoms stream from the scrubbing zone, there may be some magnesium sulfite formed from the scrubbing operation. It should also be pointed out that there may be some portion of calcium bisulfite present in the slurry stream and that the terminology of a "calcium sulfite-containing stream" is used in the broad sense to the extent that it is believed that most of the sulfur-containing ions will be present in the form of calcium sulfite.
Typically, in connection with the present improved system, there will also be the addition of sulfuric acid or other acidic medium to provide a low pH in the withdrawn slurry stream in order to enhance the oxidation reaction to form calcium sulfate. In other words, sulfuric acid will be added to the withdrawn slurry ahead of aerator-ozonator zone, or to such zone, such that there is a low pH condition in the conversion zone. Actually, various acidic mediums may be utilized to lower the pH of the slurry stream; however, sulfuric acid is preferable in order to preclude adding other than sulfate ions into the solution.
With regard to the ozone-containing stream which is to be charged into the aerator-ozonator zone, it is not intended to limit the present invention to any one method or means for preparing ozone inasmuch as it is contemplated that the ozone will be prepared in a conventional commercial manner. For example, ozone may be produced by the electronic irradiation of air or oxygen. Also, ozone may be formed by the passage of air or oxygen through an ultraviolet light source. Premanufactured ozone may be shipped to the ozonation cite; however, typically, ozone is manufactured adjacent the site where it is to be utilized by a conventional ozone production means and the problems of ozone shipment eliminated.
Most stack gas treating systems which incorporate flue gas scrubbing will also make use of an electrostatic precipitator to remove fly ash ahead of the scrubbing step and as a result there is a minimum of fly ash material within the typical countercurrent scrubbing operation. In view of this type of typical system, it is also within the scope of the present invention to utilize at least a portion of the gas stream which has passed through an electrostatic precipitator for fly ash removal inasmuch as ozone is produced in the precipitator and such electric discharge treated stream may well be utilized to pass through the ozone producing unit or optionally be utilized as is to combine with ozone being separately produced and then flow in a combined stream to the aerator-ozonator zone. It is not intended to limit the present invention to requiring that any given quantity of ozone shall be present in the aerating stream; however, since it is recognized that ozone is a more powerful oxidizing agent than oxygen, it will be desirable to have as much ozone present in the oxidizing stream as is economically feasible from the ozone producing equipment.